The present invention provides an ultra-rapid annealing treatment for both regular and high permeability grain oriented electrical steel prior to decarburizing to provide a smaller secondary grain size and lower core loss after the final high temperature anneal.
Electrical steels having up to 6.5% silicon have a final grain size and texture which determines the magnetic properties of the material. The grain size and texture will depend on the annealing temperatures, percent reductions, atmospheres, times and inhibitor systems used in the production of the electrical steel. For purposes of an exemplary showing, the invention will be applied to cube-on-edge oriented electrical steel having the (110)[001] orientation as designated by the Miller's indices. Grain oriented electrical steels are normally referred to as either regular grain oriented or high permeability grain oriented. Regular grain oriented grades generally have a permeability at 796 A/m of less than 1870 whereas high permeability grades have a permeability greater than 1870. U.S. Pat. No. 3,764,406 is typical of regular grain oriented electrical steel and U.S. Pat. Nos. 3,287,183; 3,636,579; 3,873,381 and 3,932,234 are typical of high permeability grain oriented electrical steel. The objective is to provide a steel capable of preferentially forming and sustaining the growth of (110)[001] oriented secondary grains, thereby providing these electrical steels with a sharp (110)[001] texture. The above patents teach typical routings for casting a melt composition into ingots or slabs, hot rolling, annealing, cold rolling in one or more stages, subjecting the cold rolled strip to an annealing treatment which serves to recrystallize the steel, reduce the carbon content to a nonaging level and form a fayalite surface oxide, coating the annealed strip with a separator coating and subjecting the strip to a final high temperature anneal within which the process of secondary grain growth occurs. A forsterite or "mill" glass coating is formed by reaction of the fayalite layer with the separator coating. Secondary grain growth occurs during the final high temperature anneal, but the prior processing stages establish the proper distribution of grain growth inhibitors and the texture required for secondary grain growth.
To increase the percentage of crystals having the preferred (110)[001] orientation, U.S. Pat. No. 2,965,526 used heating rates of 1600.degree. C. to 2000.degree. C. per minute (50.degree. F. to 60.degree. F. per second) to recrystallize oriented electrical steel strip between two stages of cold rolling. The intermediate recrystallization anneal was conducted at a soak temperature of 850.degree. C. to 1050.degree. C. (1560.degree. F. to 1920.degree. F.) for less than one minute to avoid undue crystal growth. The strip is again cold rolled and given a second rapid anneal, heating at 1600.degree. C. to 2000.degree. C. per minute (50.degree. F. to 60.degree. F. per second) and held at a temperature of 850.degree. C. to 1050.degree. C. (1560.degree. F. to 1920.degree. F.) to soften the material for a period of less than one minute. After the second rapid anneal, the material is decarburized at 600.degree. C. to 800.degree. C. (1110.degree. F. to 1470.degree. F.) in wet hydrogen and given a final high temperature anneal at 1000.degree. C. to 1300.degree. C. (1830.degree. F. to 2370.degree. F.). The rapid heating rates were believed to cause the strip to pass quickly through the temperature range within which undesirable crystal orientations grow and to attain a temperature within which the preferred crystal orientations grow.
U.S. Pat. No. 4,115,161 used a similar rapid heat treatment during the heating stage of the decarburizing anneal for boron-inhibited silicon steels which were stated to have processing characteristics unlike conventional silicon steels. The proper heating rate was stated to improve magnetic properties by allowing the use of a more oxidizing atmosphere during the decarburizing anneal without incurring unduly high loss of boron during the anneal. The cold rolled strip was rapidly heated from 833.degree. C. to 2778.degree. C. per minute (225.degree. F. to 82.degree. F. per second) to a temperature of 705.degree. C. to 843.degree. C. (1300.degree. F. to 1550.degree. F.). The strip was held at temperature for at least 30 seconds, and preferably for 1-2 minutes, to minimize boron lost at the surface while reducing the carbon content to less than 0.005% and providing a surface oxide scale capable forming a higher quality forsterite, or mill glass, coating after the subsequent high temperature anneal.
A Russian article by Szymura and Zawada, "Effect of the Heating Rate During Primary Recrystallization on the Properties of the Fe-3 Percent Si Alloy After Secondary Recrystallization", Arch. Hutn., 1978, 23, (1), pages 29-33, studied the influence of heating rate during primary recrystallization of cold rolled electrical steel. Electrical steel strip was hot rolled, decarburized, initially cold rolled, intermediate annealed, finally cold rolled and subjected to primary recrystallization annealing using heating rates from 1.2.degree. C. to 180,000.degree. C. per minute (0.04.degree. F. to 5400.degree. F. per second) to a temperature of 950.degree. C. (1740.degree. F.) in a dry hydrogen atmosphere, after which the strip is subjected to a high temperature final anneal to induce secondary grain growth. The magnetic properties produced during this study were not acceptable for regular grain oriented requirements. The optimum texture was developed at 50.degree. C. per second (90.degree. F. per second). Heating rates above 100.degree. C. per second (180.degree. F. per second) drastically reduced the texture. The Russian theory proposed the heating rate formed a greater number of (110)[001] nuclei during primary recrystallization. A smaller secondary grain size was believed to result from the increased number of nuclei. However, the steelmaking process of this article differs considerably from the generally accepted art wherein the decarburizing step is conducted on cold rolled strip prior to the final anneal.
It is important to note that the ultra-rapid anneal of the present invention heats the entire strip and should not be confused with the techniques of local radio frequency induction heating or resistance heating for domain refinement such as taught by U.S. Pat. No. 4,545,828 or U.S. Pat. No. 4,554,029. In U.S Pat. No. 4,545,828, the local treatment causes the primary grains to grow at least 30-50% larger than the untreated bands to act as temporary barriers to secondary grain growth and which are eventually to be consumed by the growing secondary grains. In U.S. Pat. No. 4,554,029, the material has already been given the final high temperature anneal before the locally heated treated bands have the microstructure altered to regulate the size of the magnetic domains after a further high temperature anneal.